1) Field of the Invention
The present invention relates to rough biaxially oriented polyester films which contain finely divided, inorganic and/or organic particles, have good dielectric properties and are therefore very suitable for the production of capacitors. The present invention also relates to a polyester raw material used to make such a film. The invention relates in particular to polyester films which are suitable for the production of oil-impregnated capacitors.
2) Prior Art
Oil-impregnated capacitors are employed when high voltages are used, as required in the case of power capacitors. The films used for this purpose have a greater film thickness than those used, for example, in component electronics. The thickness range of films for the production of oil-impregnated capacitors is usually from 2.5 to 30 .mu.m. For the stated applications, the films may be metallized or may be present as a film/foil laminate.
Oil-impregnated capacitors generally have the following advantages over capacitors not impregnated with oil
the capacitance of the capacitor increases owing to the higher dielectric constant of the oil in comparison with air or a vacuum, PA1 the capacitance drift of oil-impregnated capacitors is smaller, with the result that the life of these capacitors is longer, and PA1 the operating voltages of oil-impregnated capacitors are generally higher. PA1 be economically producible, PA1 have a high dielectric strength, PA1 have a roughness R.sub.z of &gt;1.0 .mu.m (R.sub.a &gt;0.1 .mu.m) PA1 be capable of being processed without creases and strains, PA1 have good oil impregnability, and PA1 have good long-term electrical properties. PA1 at least on one film surface, the roughness value R.sub.z is greater than 1.0 .mu.m and the roughness value R.sub.a is greater than 0.1 .mu.m, PA1 the AC dielectric strength is greater than 220 kV/mm (kilovolts/millimeter) and the DC dielectric strength is greater than 500 kV/mm, and PA1 the migrating fraction, based on the mass of the added particles in a polyester volume of 100 cm.sup.3 is less than 18 .mu.S/cm.
According to the prior art, rough biaxially oriented polypropylene films are advantageously used for the production of oil-impregnated capacitors, owing to their low dielectric loss factor and their high dielectric strength.
The production of oil-impregnated capacitors with metallized plastics films or with film/foil laminates is most successful when the films can be readily wetted with oil or the windings or layer packets of layer capacitors can be readily impregnated with oil.
For good wetting of the film with oil and for good impregnation of the winding/capacitor, it is advantageous if the films used have a greater roughness than standard films (for example, packaging films, magnetic tape films or process films). In general, the roughness of standard films is of the order of magnitude of a few nanometers (nm), whereas roughnesses of the order of magnitude of a few microns (.mu.m) are required for oil-impregnated capacitor films. If a standard film having little roughness is used for the impregnation of the capacitor with oil, the air entrained during winding of the films is only incompletely removed by the impregnating oil. The air remaining in the capacitor promotes local corrosion effects due to overvoltages, with the result that the dielectric strength and the capacitance of the capacitor are substantially reduced.
Suitable raw materials for film-based capacitors are in particular polypropylene and polyester. Impregnable, biaxially oriented polyester films are used for the production of capacitors in particular when higher heat stability or higher mechanical strength is required than with the use of polypropylene films. On the other hand, polypropylene has the advantage that great film roughnesses (in the .mu.m range) can be obtained by suitable process engineering measures. Use is made of the fact that polypropylene can crystallize in two different crystal modifications which, when they are present side by side in the film, lead to increased roughness. In the production of biaxially oriented polyester films, roughnesses as high as polypropylene films cannot be obtained by process engineering measures.
According to the prior art, in the case of biaxially oriented polyester films, the great film roughness required for winding, further processing and impregnation is achieved by means of the addition of suitable inorganic and/or organic particles or particle systems in certain concentrations.
A document which describes this procedure for winding or for further processing of films is U.S. Pat. No. 3,980,611. According to the teaching of this publication, the film roughness and the film handling are improved by a combination of small (smaller than 1 .mu.m), medium (from 1 to 2.5 .mu.m) and large (from 2.5 to 10 .mu.m) particles, it being necessary to fulfill certain ratios of film thickness to particle concentrations.
According to JP-A 03 917 136, films or laminated films having high dielectric strengths and small electrical capacitance drifts in the corresponding components are produced by the use of small particles, such as colloidal silica.
According to JP-A 63 141 308, from 1.4 to 16 .mu.m thick films having good processibility and high dielectric strengths are produced by the combination of silicone resin particles with other inert particles.
According to JP-A 03 246 814, crosslinked polymer particles may also be used for capacitor film applications, and according to JP 88 033 290 inert particles having a molar P/Ca ratio of from 0.7 to 2.5 may also be used.
Furthermore, EP-A 0 423 402 discloses that high particle concentrations or large particle diameters lead to a greater level of vacuoles around the particles during orientation with the usual orientation ratios, owing to a relatively poor affinity between particle and polymer matrix. Corresponding vacuoles have an adverse effect on the mechanical film properties and on the dielectric strength of the films. In addition, a larger number of tears are caused during film production, leading to a deterioration in the productivity during film production.
Another method of obtaining rough polyester films having excellent processing properties, very good oil impregnability and very good electrical properties is described in JP-A 63 72 531. This document describes the production of rough oil-impregnable 12 .mu.m thick films by an additional coating step in which a "swellable" silicate is applied.
All rough polyester films known to date have the disadvantage that the electrical properties of these films are poorer than the electrical properties of the polyester used for their production. This indicates that the electrical properties are impaired by the incorporation of additives, particles and other auxiliaries to the polyester raw material.